Material handling vehicles, such as industrial lift trucks, frequently must be docked at a particular location with respect to a loading/unloading station. In the case of operator controlled vehicles this merely requires ordinary skill on the part of the vehicle operator. However, in the case of operatorless automatic guided vehicles, the process involved in accurately docking the vehicle is considerably more complex.
In a typical situation, an automatic guided vehicle approaches a docking or stop location at a normal travel speed. Upon approaching the desired docking point, the vehicle automatically slows to a reduced approach speed. Finally, upon actually reaching the docking point, the vehicle is brought to a halt.
This seemingly simple procedure is complicated by various external factors. For example, common automatic guided vehicles utilize electrical braking as an energy efficient means for stopping the vehicle. Electrical braking, while effective as a means for slowing a vehicle over a relatively long period of time, is less than ideal for making instantaneous corrections in vehicle velocity when the vehicle is moving at a relatively slow speed. Electrical braking requires switching the direction contactors typically utilized in the control circuit of an electrical vehicle from forward to reverse configurations. The switching procedure requires a determinable amount of time that necessarily slows the response of the electrical braking.
Rapid response of the braking system is particularly necessary where the surface condition over which the vehicle must travel is rough or uneven. Since electrical braking requires considerable time to respond, such unevenness cannot be readily compensated for without some alternative means for stopping the vehicle. The selected compensation method must not inhibit travel of the vehicle up small rises in the travel surface, but must be capable of inhibiting gravity induced "coasting" down similar irregularities in the travel surface.
The present invention is directed to overcoming one or more of the problems as set forth above.